DESCRIPTION (Applicant's Abstract): The T-cell receptor (TCR) recognizes not only cognate ligands consisting of an antigenic peptide complexed with MHC molecules, but also low affinity ligands formed by self peptide/MHC complexes. Recognition of low affinity ligands by the TCR is required for: a) positive selection of thymocytes, which produce a repertoire of T-cells bearing useful TCR specificities, and b) maintenance and survival of peripheral T-cells. However, little is known about how these low affinity TCR-ligand interactions induce molecular changes within the cell that result in these biologically important responses. The goal of this study is to understand how recognition of low affinity ligands can induce gene transcription important for these responses. The zinc-finger transcriptional activator early growth response gene 1 (Egr-1) has been implicated in the response to low affinity TCR ligands by virtue of its expression in T-cells and thymocytes in response to TCR ligation, and by a change in the threshold for positive selection in mice that over-express Egr-1. Furthermore, in non-lymphoid cell types, Egr-l is known to play a role in inducing differentiation in response to extracellular stimuli. The co-repressor Nab-2 regulates the transcriptional activation mediated by Egr- 1. This evidence leads to the hypothesis that Egr- 1 and Nab-2 are two genes that are regulated by TCR interaction with low affinity ligands, resulting in a temporal window in which new gene transcription is activated that leads to positive selection and T-cell survival. Using a TCR transgenic system that has numerous ligands covering a range of TCR affinities, we will define the regulation of Egr-1 and Nab-2 by high and low affinity TCR ligands. This will include analysis of the factors that bind to the Egr-l and Nab-2 promoters, and the signaling pathways that regulate these factors. In addition, the specific roles that Egr- 1 and Nab-2 play in positive and negative selection, T-cell homeostasis, and peripheral T-cell differentiation will be defined using mice that contain targeted deletions in the genes for Egr-l or Nab-2. The results of these experiments will contribute significantly to a molecular understanding of T-cell responses to low affinity ligands.